Sampling and analytical methods
The bioreactor was monitored online using measurements of the pH, and
base addition. The biogas was measured qualitatively for
CO2, H2 and CH4 in-line
with a mass spectrometry system (Omnistar, Pfeifer vacuum). Offline
measurements were done for SVI, soluble chemical oxygen demand
(CODSol), glucose concentration, organic acids
concentration, total suspended solids (TSS) and volatile suspsended
solids (VSS). The VSS content of the reactor was determined by taking a
VSS sample at the end of the reaction phase, when the reactor was
completely mixed. The reactor was sampled in more detail at the end of
the enrichment elucidating the conversions during a cycle. To this end
the reactor was sampled every 5 min during the presence of glucose and
once every 30 min after glucose depletion. Samples taken for
CODSol glucose, and organic acid analysis were filtered
directly after sampling using a 0.45 µM pore size filter (PVDF membrane,
Millipore, Ireland). The CODSol was measured using
commercially available spectrophotometrically test-kit from Hach-Lange.
The organic acid composition and quantity were measured using a
high-performance liquid chromatography with a BioRadAminex HPX-87H
column and a UV/RI detector (Waters 2489). As a mobile phase 1.5 mM
H3PO4 in Milli-Q water was used with a flow rate of 0.6
mL·min-1 and a temperature of 60°C. The TSS content of
a sample was obtained by first centrifuging a sample at 2500 g after
decanting the supernatant the sample was oven dried at 105 ºC. The VSS
content was obtained according to (Clesceri et al. 1999). The SVI60 was
obtained by taking the apparent sludge volume (SV) at the end of the
settling phase in the bioreactor and dividing this by the measured TSS
from the mixed reactor.